This invention relates to microstrip antenna devices, and finds particular application in the UHF reception of satellite television signals.
As is known, a microstrip antenna device has a radiating structure comprising:
a dielectric layer, a conductive patch of a chosen shape on one side of the dielectric layer, and on the other side of the conductive layer a conductive plate forming a ground plane, and PA1 a feed circuit defining the mode of feeding UHF energy to the radiating structure and comprising, for example, at least one microstrip line connected to the conductive patch and implanted in the dielectric layer in the same plane as the conductive patch.
A person skilled in the art will know that, for a given dielectric constant, the geometric dimensions of the conductive patch determine the centre operating frequency of the antenna device, and that the thickness of the said dielectric layer is substantially proportional to the usable frequency bandwidth.
Thus, in order to keep a predetermined usable frequency bandwidth, it is necessary to determine a corresponding thickness of the dielectric layer.
The usable frequency bandwidth, in the field of telecommunications antennae, necessitates a thickness of the dielectric layer such that, if it is intended to produce microstrip lines with an impedance of 50 ohms, the width of the latter is significant, for example a width of 20 mm for a thickness of 5 mm of air.
The use of such a line width gives rise to drawbacks, in particular in the implantation of the feed distributor when the antenna device comprises a network of conductive patches as well as in the production of the resistive couplers in microstrip feed lines. It also gives rise to a greater amount of spurious radiation.
One solution in such a case consists of working with a line impedance of a value above 50 ohms, for example 70 ohms, but it has the disadvantage of requiring adaptor sections which are cumbersome and difficult to use.
The invention provides a satisfactory solution to this problem in a microstrip antenna device of the type described above.